Process for increasing the resistance to corrosion of light metals and light metal alloys



- temperature.

Patented Jan. 21, 1941 UNITED STATES PATENT OFFICE Josef Martin Michel Bitterfeld, Germany,

and Fritz Henneberger,

ass'ignors, by mesne assignments, to Magnesium Development Corporation, a corporation of Delaware No Drawing.

rial No. 165,076. 1936 6 Claims.

This invention relates to a process for increasing'the resistance to corrosion-of workpieces of light metals and light metal alloys.

Numerous processes are known for the production of layers containing fluorine, on light metals and light-metal alloys, for the purpose of increasing their resistance to corrosion.

Merely dipping the workpieces into aqueous solutions of fluorides furnishes layers of extreme tenuity, which are porous and provide inadequate protection against corrosive influences.

It has also been recommended to produce fluoride layers on light-metal alloys by the electrolysis of aqueous solutions containing fluorine, such as of hydrofluorid acid, and also of fused fluorine-containing salts such as acid or complex fluorides, polyfluorides and ammonium bifluoride+urea. Some of these electrolytic processes furnish quite useful coatings, but they are all attended with the drawback that elevated working temperatures must be employed. Since, under such working conditions, the aforesaid fluorine compounds already havean appreciable hydrogen fluoride vapour pressure, losses of substance can never be completely avoided. Moreover, at the elevated working temperatures, the material under treatment becomes att ked to a substantial extent. It has been propose to prevent vaporisation losses, and the corrosion of the workpieces by hydrofluoric acid vapours, by forming a protective coating on the surface of the electrolyte by means of fused hydrocarbons of high molecular weight which are immiscible with the fluoride melt. This, however, is attended with considerable difficulties in practical application.

The present invention aims at providing a process for the production of protective fluoride layers, of satisfactory mechanical and anti-corrosive properties, on light metals, especially magnesium and magnesium alloys, by anodic treatment at ordinary temperatures.

To this end, according to the present inven@ tion, neutral alkali metal fluorides are employed in alcoholic solution as electrolyte at ordinary The preferred solvents used in carrying out the invention are polyhydric alcohols, such as glycerol or glycol, either singly or jointly. Mixtures of polyhydric alcohols with monohydric alcohols are also suitable for the purposes of the invention. Such mixtures may also contain a certain proportion of water when employed as solvents of neutral alkali metal fluorides. In order to improve the adhesion and compactness of the resulting anodic layer, it

Application September 22, 1937, Se-

Germany September 25,

has been found advisable to maintain a pH value exceeding 8 but not exceeding 11 in the solutions, as has already been proposed for inhibiting the corrosive action of alcohols on light-metal alloys.

In producing fluoride layers in purely alcoholic solutions, a voltage of at least 50 is necessary, and this can be increased up to 150 volts,

whilst, in the case of mixtures of alcohols and water, a working voltage below 50 may be employed. The current density is about 1 amp. per sq. decimetre at the outset, but falls very quickly as the operation proceeds, and flnally becomes a mere fraction (about V of the initial value. The length of the treatment when using an alcohol ora mixture of alcohols as solvent, does not exceed minutes; but in the case of mixtures of alcohol and water, up to 30 minutes are generally required.

In carrying out the treatment direct current is preferably employed, but use can also be made of twoor poly-phase alternating current or of superimposed, direct and alternating current, or

of pulsating direct current.

Example I The workpiece to be treated, such as a magnesium alloy containing 1% of zinc, 6% of aluminiumand 0.5% of manganese, is suspended as anode in an electrolyte consisting of a solution of 100 grms. of potassium fluoride in 1 litre of ethylene glycol, and a high percentage magnesium alloy is employed as cathode. A direct current of 100 volts is passed through the bath'at room temperature (20 to C.) the initial current density amounting to 1 amp. per sq. decimetre of surface. At the end of five minutes a coherent, firmly adherent layer of fluoride has become deposited on the surface of the anode. If desired, this layer can be consolidated by suitable known methods.

Example II We claim:

1. Process for increasing the resistance to corrosion of workpieces of magnesium and magnesium base alloys-by formation of a protective layer on such' workpieces which comprises employing the workpiece as the anode in an electrolytic treatment at room temperature with an: electrolyte consisting of a solution of a neutral alkali fluoride in a solvent comprising predomi nantly an alcohol. a

2. Process for increasing the resistance to corrosion of workpieces of magnesium and magnesium base alloys by formation ofa protective layer on such workpieces which comprises em! ploying the workpiece as the anode in an electrolytic treatment at room temperature with an elec-.- trolyte consisting of a solution of a neutral alkali fluoride in a solvent comprising predominantly a polyhydric alcohol. s

3. Process for increasing the resistance to corrosion of workpieces of magnesium and 'magnesium base alloys by'formation of a protective layer on such workpieces which comprises employingthe workpiece as the anode in an electrolytic treatmentat room temperature with an electrolyte consisting of a solution of a neutral alkali fluoride in a solvent comprising predominantly glycol.

4.- Process-for increasing the resistance to corrosion of workpieces of magnesium and mag- -,alcohol, the least 7'.

nesium base alloys by formation of a protective layer on such workpieces which comprises employing the workpiece as the anode in an electrolytic treatment at room temperature with an electrolyte consisting of a solution of a neutral alkali fluoride in a solvent comprising predominantly an alcohol and also water.

,-5. Process for increasing the resistance to corrosion of workpieces of magnesium and mag- [nesium base alloys by formation of a protective layer on such workpieces which comprises employing the workpiece as the anode in an electrolytic treatment atroom temperature with an electrolyteconsisting of a solution of a neutral alkali fluoride in a'solvent comprising prodominantly an JOSEF MARTIN MICHEL. FRITZ HE'N'NEBERGER.

pH value of the electrolyte being at 

